1. Field of the Invention:
The present invention relates to a semiconductor device having a high density formed at a high speed, and more specifically, it relates to a method for manufacturing a semiconductor device in which a metal silicide film is selectively formed on an Si substrate.
2. Description of the Related Art:
In recent years, with the miniaturization of semiconductor devices, the necessity of decreasing the depth of junctions in a diffusion layer has been strengthened more and more. However, the decrease of the junction depth causes the resistance of the diffusion layer to increase. Thus, as a technique for avoiding this disadvantage, a salicide process has been used in which a diffusion layer region alone is selectively made from a high-melting point metal silicide.
Typical examples of the high-melting point metal which can be used in this salicide process include Ti, Ni and Co. Among others, cobalt silicide has about the same lattice constant as that of Si and is excellent in lattice matching properties, and for this reason, cobalt silicide is expected as a high-melting point metal silicide for constituting a salicide structure in place of Ti silicide.
As a technique for improving the heat resistance of cobalt silicide, there has been a method which comprises making the solid phase epitaxial growth of cobalt silicide from a silicon substrate. However, it has been known that if cobalt is directly grown on the silicon substrate, polycrystallization occurs instead of the epitaxial growth.
The causes of this phenomenon are considered to be that if a heat treatment is carried out while cobalt is in direct contact with silicon, cobalt silicide which is not a disilicide is formed as an intermediate layer before a temperature for the heat treatment has been reached, and that a spontaneous oxide film of silicon remains in an interface between cobalt and silicon ["Journal of Applied Physics", Vol. 70, pp. 7579-7587 (1991)].
This spontaneous oxide film can be removed by a method which comprises sputtering titanium prior to the sputtering of cobalt onto the substrate to react the spontaneous oxide film present in the interface with Ti. This method utilizes means in which deposition is done so as to obtain a Co/Ti/Si laminate structure and a heat treatment is then carried out [e.g., "Journal of Applied Physics", Vol. 72 (5), pp. 1864-1873 (1992)].
In the case that this method is employed, a facet of cobalt silicide is observed in the interface between epitaxially grown cobalt silicide and the silicon substrate. The step of the interface attributed to this facet can be eliminated by a technique which comprises sputtering titanium onto the silicon single crystal substrate, sputtering cobalt thereonto, and further repeating this sputtering operation of titanium and cobalt plural times as shown in FIG. 1 to form a multiple laminate structure [e.g., "Applied Physics Letters", Vol. 61, pp. 1519-1521 (1992)]. However, this method has a problem that the process is complex.
Furthermore, as alternate means, Japanese Patent Application Laid-open No. 166752/1993 has described a method which comprises forming an oxide film on a silicon substrate by the thermal oxidation of silicon, and then forming titanium silicide. This method will be described with reference to FIG. 2.
As shown in FIG. 2A, there is formed an MOS transistor having an LDD structure which comprises a gate electrode 3 consisting of tungsten silicide 14 and polysilicon 15, an element separating region 6, a diffusion layer 5 and a sidewall 4. Next, a silicon film having a thickness of 3 nm is deposited all over, and a silicon oxide film 7 is then formed by the thermal oxidation.
Next, a titanium layer 8 having a thickness of about 30 nm is deposited all over [see FIG. 2B]. Afterward, RTA (rapid thermal annealing) which is a low-temperature thermal treatment is carried out at 650.degree. C. for 30 seconds by the use of an inert gas to convert the titanium layer 8 into titanium silicide. Next, the laminate is immersed in a mixed aqueous solution of ammonia and hydrogen peroxide for 10 minutes to selectively etch and remove titanium which has not been converted into the silicide. According to this method, the diffusion layer region can be selectively converted into the silicide [see FIG. 2C].
In this disclosed method, the silicon oxide film is exclusively used for the purpose of forming a titanium silicide layer 16. In the case that cobalt is formed on the oxide film, such a silicide is scarcely formed.